That's in line with a disturbing trend: the space agency said 9 out of the 10 hottest years in the modern meteorological record have occurred since the year 2000. The other year in the top 10 was 1998.

"We know the planet is absorbing more energy than it is emitting," said Goddard Institute for Space Studies director James E. Hansen. "So we are continuing to see a trend toward higher temperatures. Even with the cooling effects of a strong La Nina influence and low solar activity for the past several years, 2011 was one of the 10 warmest years on record."

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NASA says it gathers weather from 1,000 meteorological stations around the world, satellite readings of the sea's surface temperature, and recordings taken in Antarctica.

The space agency attributes rising temperatures to increased levels of greenhouses gases, like carbon dioxide. According to NASA, carbon dioxide measured in the atmosphere rose from 285 parts per million in 1880, to 390 parts per million today.

NASA says that even warmer global average temperatures could be on the way in the next few years, because of increased solar activity and an El Nino affect that will raise Pacific temperatures.